a. Field of the Invention
Broadly speaking, this invention relates to telecommunications. More particularly, in a preferred embodiment, this invention relates to methods and apparatus for interconnecting time and frequency-division multiplex carrier systems.
B. Discussion of the Prior Art
As is well known, the communications industry makes extensive use of frequency-division multiplex carrier systems. However, recent advances in silicon technology have made the use of time-division multiplex systems increasingly attractive, especially for short-haul use.
Because of the large investment in existing plant, both FDM and TDM carrier systems will co-exist in industry for years to come. It thus becomes necessary to consider how best to interface these systems, as might be required, for example, at some intermediate central location.
One approach to this problem is simply to convert the signals carried by both systems to voiceband and then make the necessary interconnexion on a channel-by-channel, voice-frequency basis. This requires a back-to-back connexion of existing analog equipment which, so far, has been avoided for economic reasons.
A paper by Freeny, Kieburtz, Mina and Tewksbury entitled "Design of Digital Filters for an All Digital Frequency Division Multiplex-Time Division Multiplex Translator" which appeared in the IEEE Transactions on Circuit Theory, Vol. CT-18, No. 6, Nov., 1971, pp. 702-711, and a paper by C. F. Kurth entitled "SSB/FDM Utilizing TDM Digital Filters" which appeared in IEEE Transactions on Communication Technology, Vol. COM-19, No. 1, Feb., 1971, pp. 63-71, both suggest that the interface between an analogue multiplex system and a digital multiplex system may be effectuated in an alldigital manner by the use of digital filtering.
The above-referenced papers propose several possible approaches to this problem and demonstrates that the all-digital approach is economically viable. However, no practical solution is given in these papers.